Quantum many-body spin rings coupled to ancillary spins: The sunburst quantum Ising model

TL;DR

This paper investigates the ground-state properties of a quantum Ising spin-ring coupled to ancillary spins, revealing different regimes and phase transition behaviors depending on system parameters and size scaling methods.

Contribution

It introduces the 'sunburst model' combining a quantum Ising ring with ancillary spins, analyzing its phase transitions and size-scaling effects using renormalization-group and finite-size scaling.

Findings

Rapid nonanalytic changes near quantum transitions for small ancillary energy and coupling.

Smoother behaviors in the disordered phase with fixed ancillary energy.

Ancillary spin effects scale proportionally to the square root of their number for large systems.

Abstract

We study the ground-state properties of a quantum "sunburst model", composed of a quantum Ising spin-ring in a transverse field, symmetrically coupled to a set of ancillary isolated qubits, to maintain a residual translation invariance and also a $\mathbb{Z}_2$ symmetry. The large-size limit is taken in two different ways: either by keeping the distance between any two neighboring ancillary qubits fixed, or by fixing their number while increasing the ring size. Substantially different regimes emerge, depending on the various Hamiltonian parameters: for small energy scale $\delta$ of the ancillary subsystem and small ring-qubits interaction $\kappa$, we observe rapid and nonanalytic changes in proximity of the quantum transitions of the Ising ring, both first-order and continuous, which can be carefully controlled by exploiting renormalization-group and finite-size scaling frameworks. Smoother behaviors are instead observed when keeping $\delta>0$ fixed and in the Ising disordered phase. The effect of an increasing number $n$ of ancillary spins turns out to scale proportionally to $\sqrt{n}$ for sufficiently large values of $n$.